39 U.S. Code § 3208 - Shipment by most economical means

Shipments of official matter other than franked mail shall be sent by the most economical means of transportation practicable. The Postal Service may refuse to accept official matter for shipment by mail when in its judgment it may be shipped by other means at less expense, or it may provide for its transportation by freight or express whenever a saving to the Government of the United States will result therefrom without detriment to the public service.